A hydraulic excavator or other construction machine driven by a hydraulic system often includes a large-size engine that is selected for operations performed at maximum load and capable of performing all sorts of operations ranging from light-load operations to heavy-load operations. However, the heavy-load operations (e.g., heavy excavating work performed by the hydraulic excavator for frequent excavating and loading of earth) performed by the construction machine account for only a part of all operations performed the construction machine. Therefore, even if the construction machine includes such a large-size engine, its capacity is not fully used during a light-load operation or a medium-load operation (e.g., light excavating work performed by the hydraulic excavator for earth surface leveling). This problem should be avoided from the viewpoint of fuel consumption reduction. There is a known hybrid construction machine that addresses the above problem by employing a small-size engine to reduce fuel consumption and by providing assist with a motor generator that compensates for output deficiency due to the use of the small-size engine.
A technology concerning a hybrid construction machine is described, for instance, in JP-A-2007-218111. This technology is designed to provide an operator with an improved operation feeling when an engine running at a low speed is rapidly accelerated, for instance, to promptly resume an operation from an idle state. A control device for the hybrid construction machine, which incorporates the above technology, includes determination means for determining, in accordance with a target rotation speed of the engine (motor generator), an actual rotation speed of the motor generator, and the amount of electric power remaining in an electrical storage device, whether an assist output generated by the motor generator is required. If the determination means determines that the generation of the assist output is not required, a first maximum torque line is selected as a maximum torque line indicative of a maximum absorption torque that can be absorbed by a hydraulic pump. The first maximum torque line increases a maximum absorption torque when the target rotation speed of the engine increases. If, on the other hand, the determination means determines that the generation of the assist output is required, a second maximum torque line is selected as the maximum torque line. The second maximum torque line provides a higher maximum absorption torque in a low rotation speed region of the engine than the first maximum torque line. This ensures that when the motor generator generates the assist output, the torque absorbed by the hydraulic pump during an increase in the engine's rotation speed is higher than that when the motor generator does not generate the assist output. Consequently, the construction machine starts moving earlier in response to the movement of an operating lever, thereby reducing an uncomfortable operation feeling given to the operator.